Historically, solar power (both in space and terrestrially) has been predominantly provided by silicon solar cells. In the past several years, however, high-volume manufacturing of high-efficiency III-V compound semiconductor multijunction solar cells for space applications has enabled the consideration of this alternative technology for terrestrial power generation. Compared to silicon, III-V compound semiconductor multifunction cells are generally more radiation resistant and have greater energy conversion efficiencies, but they tend to cost more to manufacture. Some current III-V compound semiconductor multijunction cells have energy efficiencies that exceed 27%, whereas silicon technologies generally reach only about 17% efficiency. Under concentration, some current III-V compound semiconductor multijunction cells have energy efficiencies that exceed 37%.
Generally speaking, the multijunction cells are of n-on-p polarity and are composed of a vertical stack of InGaP/(In)GaAs/Ge semiconductor structures. The III-V compound semiconductor multijunction solar cell layers are typically grown via metal-organic chemical vapor deposition (MOCVD) on germanium (Ge) substrates. The use of the Ge substrate permits a junction to be formed between n- and p-type Ge, thereby utilizing the substrate for forming the bottom or low band gap subcell. The solar cell structures are typically grown on 100-mm diameter Ge wafers with an average mass density of about 86 mg/cm2. In some processes, the epitaxial layer uniformity across a platter that holds 12 or 13 Ge substrates during the MOCVD growth process is better than 99.5%. The epitaxial wafers can subsequently be processed into finished solar cell devices through automated robotic photolithography, metallization, chemical cleaning and etching, antireflection (AR) coating, dicing, and testing processes. The n- and p-contact metallization is typically comprised of predominately Ag with a thin Au cap layer to protect the Ag from oxidation. The AR coating is a dual-layer TiOx/Al2O3 dielectric stack, whose spectral reflectivity characteristics are designed to minimize reflection at the coverglass-interconnect-cell (CIC) or solar cell assembly (SCA) level, as well as, maximizing the end-of-life (EOL) performance of the cells.
In some compound semiconductor multijunction cells, the middle cell is an InGaAs cell as opposed to a GaAs cell. The indium concentration may be in the range of about 1.5% for the InGaAs middle cell. In some implementations, such an arrangement exhibits increased efficiency. The advantage in using InGaAs layers is that such layers are substantially better lattice-matched to the Ge substrate.